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There's no cap. But the different forms of penetration are calculated differently.

First you'll have flat magic reduction (Abyssal Scepter, for instance, where you literally reduce the value of someone's magic resist - the main difference between reduction and penetration is that reduction can bring values into the negative.) After that you have flat magic penetration, and finally percentage magic penetration.

EDIT: To clarify, if you have Sorcerer's Boots, Abyssal Scepter and Void Staff (disregarding masteries/runes for simplicity's sake), and you're fighting someone with 100 magic resist. First, your Abyssal Scepter will reduce his magic resist to 80 (not just for you, but teammates.) Then, your spell will penetrate 20 more of his magic resist, so he'll effectively have 60 for you. Then, your Void Staff will help you penetrate 40% of that.

There's no cap other than the 'innate' cap that comes from limited itemization and item slots.

Percentage penetration and reduction use diminishing returns, so if you have 40% from a void staff and 10% from masteries you will only get 46% instead of 50%, which is, I guess, in place to prevent you from penetrating too deeply and causing too much damage.

Flat penetration and flat reduction stack additively, so if you have 20 from Sorcerer's Shoe's and 20 from Haunting Guise, you will get 40 total.

I did a quick test as xerath and found that my friend(cassiopeia) did an ADDIDTIONAL 25% damage when he had abyssal, s.shoes and voidstaff... so it acctually amplifies the damage past 0. thats what i found from real research.